An X-ray diffractometer device is already known from the prior art apparatus, type designation PW1050, marketed by Messrs. PHILIPS (I .alpha. E ANALYTICAL ALMELO, the Netherlands).
This device comprises an X-ray source, a system of collimation slits for the beam coming from the source, a sample support, arranged such that the incident beam reaches the sample at an angle of incidence equal to (.pi./2)-.theta., or, put differently: at an angle .theta. to the plane of the sample-support, a collimation slit system of the reflected beam, and a detector for detecting the number of reflected photons, of the proportional counter type.
Proportional counter must here be understood to mean a device containing gas which can be ionized by the flux of photons to be detected and supplies a signal in the form of a voltage which is proportional to the number of photons. Actually, the response of the proportional counter is only linear in a certain range of intensities. When the intensities reflected by the sample are too weak or too strong, one lands outside the linearity region of the counter.
Now, the diffractometry device already commercially available is not suitable for the intended use to characterize multi-layer samples, and also not for measuring layer widths, because of the fact that it can only operate in a certain range of angles of incidence. Thus, this apparatus is perfectly suitable for measuring mesh parameters of samples of powders of different materials, since in the case of powders, the angles of incidence do not have very high values, that is to say they are generally not located in the range of glancing angles or orthogonal incidences. The measurement of the mesh parameters is obtained by interpreting variations in the output signal of the proportional counter. These variations form peaks having an amplitude which is proportional to the number of photons received by the detector and whose distance is also characteristic of the material, more specifically of its mesh parameters. Comparing these measurements to the data contained in Classifying Tables renders it possible to determine the mesh parameters of the powder under investigation and to derive therefrom the nature of the composite material.
Measuring the mesh parameters by means of this method, using the known apparatus with the type designation mentioned in the foregoing, is founded on the Bragg relation: EQU 2d. Sin .theta.=.lambda.,
wherein .lambda.= the wavelength of the source, constant value,
.theta.= the angle between the path of the incident beam and a reticular plane of the investiagted material, PA1 d= the mesh parameter of the material forming the investigated powder, for example.
As the prior art apparatus is arranged for investigating powders, in conditions far removed from glancing incidence, the measurements performed with this apparatus can only be applied to materials having a mesh parameter d of a low value.
Now, at present it is necessary that one can characterize not only powders, but also bulky elements, for example multi-layer mirrors operating in the field of soft X rays, or thin metal, semiconducting or insulating layers, all solid materials.
For example, the said bulky element or multilayer mirror is formed by an alternation of at least two materials having different indices of refraction: a what is called heavy material and a what is called light material. The spacing between the layers is imposed by the structure of the mirror.
Characterizing this type of bulky element requires the measurement of large parameters d. Consequently, from the relation stated hereinbefore, the result is that, the wavelength of the source being fixed, only the measurement at very small angles of incidence .theta. (glancing incidence) renders it possible to obtain the characterization of materials having large parameters d, or, when layer thicknesses are measured, allows measuring of widths comprised between 1 and 300 nm.
It is not possible with the known apparatus, whose type designation has been mentioned hereinbefore, to operate in the case of very glancing incidence because of limitations of the proportional counter. In fact, with glancing incidence, the reflected intensities are very strong, and because of a saturation phenomenon, the proportional counter is outside the range of intensities in which its response is linear. Consequently, it is not possible to obtain the characterization of the intended samples, mentioned in the foregoing, using the known apparatus.
On the other hand, for solid samples such as the said multi-layer mirrors, it must be necessary that the angle of incidence (.pi./2)-.theta. of the values wherein .theta.=0 to the values wherein .theta. is still low but not zero, for example .theta.=2.degree. or .theta.=4.degree. . During this variation, the reflected intensity varies in large proportions. If, for example, the reflected intensity is within the linearity range of the proportional counter for .theta.=0.degree., it is no longer in this range, by lack of intensity for .theta.=2.degree.. Conversely, if the intensity is within the linearity range of the proportional counter for .theta.=2.degree., it is no longer in this range for .theta.=0.degree. because of the excessive increase of the reflected intensity.
A solution known to a person skilled in the art of optics of the problem created by an excessively high luminous intensity in a system, is to interpose an absorbing filter.
But, as has been stated in the foregoing, this solution is not directly applicable to the prior art apparatus, because of the fact that if the reflected intensity is within the linearity range of the counter in one of the measuring conditions, it is no longer in this range from the instant at which the conditions for the same measurement have changed.
A solution must therefore be found for the problem of interposing a given filter as a function of the photonic intensity reflected from a given sample when the measuring conditions vary during one measurement.
The solution then found renders it possible to realise measurements not only of solid samples but also of samples with large mesh parameters, as well as of samples which are simultaneously solid and have large parameters, that is to say when the parameters of the sample vary from one measurement to the other.